Directional wave receiving system



y 14, 1968 HIKARU DATE 3,383,689

DIRECTIONAL WAVE RECEIVING SYSTEM Filed NOV. 2, 1965 4 Sheets-Sheet l z g, K I cos 0 05 0 Q i l 5 him a, I 2 13 Ayu/ar Freyuency F: g. 4 2 3 v Incident ug r v opmdum 11251 wave out put 9 l/{jusfiny knob 751' '11" I INVENTOR /%"f Mr [)a 7'2:

I ATTORNEY HIKARU DATE DIRECTIONAL WAVE RECEIVING SYSTEM May 14, 1961s 4 Sheets-Sheet 2 Filed Nov. 2, 1965 INVENTOR BYMJWXZ ATTORNEE HlKARU DATE May 14, 1968 DIRECTIONAL WAVE RECEIVING SYSTEM 4 Sheets-Sheet 4 Filed Nov. 2, 1965 INVENTOR flaw-Q 027% M m, M

ATTORNEKS United States Patent Office 3,383,689 Patented May 14, 1968 DIRECTIONAL WAVE RECEIVING SYSTEM Hikaru Date, Tokyo, Japan, assignor to Japan Broadcasting Corporation, Tokyo, Japan Fiied Nov. 2, 1965, Ser. No. 506,087

Claims priority, application Japan, Nov. 11, 1964,

39/ 63,429 1 Claim. (Cl. 343113) ABSTRACT OF THE DISCLOSURE A wave receiving system having a high directivity, especially suitable for sound receiving with a sharp directivity. The invention utilizes the Doppler effect in order to obtain high directivity and expresses a very sharp lobe of receiving characteristics. One practical embodiment .of the invention comprises a plurality of wave receiving elements arranged on a substantially linear line. Each of the plurality of the receiving elements is successively scanned so as to produce a frequency modulated wave of the received sound wave by the angular frequency of the scanning. By deriving desired harmonics of the frequency modulated wave, a desired directivity or a desired sharpness of the lobe .of the directivity can be obtained. The system affords the great advantages of (1) response for a wide frequency band, (2) no side lobe characters, (3) easily adjustable directivity, and (4) high fidelity.

This invention relates to a system for receiving sound of electromagnetic waves, more particularly to a directional microphone system to receive sound waves at a high directivity by taking advantage of the Doppler effect.

Practically speaking, all conventional methods for obtaining directivity in a device for radiating or receiving sound or electromagnetic waves have been based on the principle of an interfering effect between waves having different phase angles. For instance, in the case .of a directional antenna assembly, a number of elementary members thereof are disposed in such a manner that the wave energy radiated from each of said elementary members are interfered with each other so as to give a concentration of energy in a certain predetermined direction or directions. In the case of a directional receiver assembly, phase difference between each wave received by each elementary member thereof depends on the incident angle of the wave to said elementary member, the wave length of the wave concerned, the geometrical configuration of such elementary members, etc., and the desired directivity may be obtained by adding outputs from such elementary members in a suitable manner so as to produce higher sensitivity in a certain direction.

Such known directional devices have a disadvantage in that their directivity is altered to a considerable extent if the frequency and accordingly the wave length of the wave to be dealt with is changed over a wide range. In most cases of modulated electromagnetic waves, the value .of the ratio between maximum frequency and minimum frequency of the side band spectrum (to be referred to a specific band hereinafter) is close to unity, and hence, it can be assumed that there may not be any significant difference in directivity due to a frequency difference over the entire frequency range of the desired band. Another disadvantage of directional wave radiating or receiving devices of conventional type is in that beside the desired highest sensitivity in the preselected direction (main lobe), there appear one or more high sensitivities in certain undesired directions (side lobes).

In order to eliminate such side lobes in undesired directions, a number of methods have been proposed. For

instance, a plurality of elementary members of a wave radiating or receiving device are disposed in a row while allowing higher sensitivities for elementary members located at the central portion of the row to be compared with'that of elementary members located at both end portions of the row, thereby suppressing the magnitude of the side lobes. Judging from the principles of the interference and the cause of side lobes, however, it is impossible to eliminate such side lobes completely. Furthermore, all the methods proposed heretofore for suppressing the side lobes seem to be effective for only one predetermined frequency but not effective for other frequencies.

In the case of sound waves, it is very difficult to establish a directivity which is uniform for the entire range of audio frequencies. The reason for such a difficulty is due to the fact that the specific band for most sound waves is usually very large, e.g. the maximum to minimum band ratio of a sound wave having a frequency spectrum ranging from 50 to 10,000 cycles, per second is 200. If the maximum and the minimum frequencies in the frequency spectrum of a sound wave differ vastly from each other, then conditions for interference at the maximum frequency thereof are obviously considerably different from those at the minimum frequency thereof, and accordingly the directivities at the maximum and minimum frequencies are different from each other too. Therefore, it is well known that in most conventional directional acoustic devices, the directivity at low frequencies is not sharp, and the sharpness of the directivity of the main lobe increases and at the same time a plurality of side lobes become apparent as the frequencies increase. Under such conditions, suppression of side lobes over the entire range of audio frequencies is extremely difficult, and in fact, there has not been obtained any device which is completely free from side lobes.

The directivity of most conventional acoustic devices, such as a coneshaped loud speaker, 21 horn-shaped loud speaker, paraboroidal'sound wave collecting means, tubular sound wave collecting means, etc., is not constant for different frequencies, but it is restricted as described in the foregoing due to its basic principles of wave interference. An exceptional microphone has been developed which is provided with a substantially constant but simple directivity over most of the range of the audio frequencies. However, even with such directional microphones, the fiatness in the directivity is lost when the frequency increases over several kc. s. in most cases.

As mentioned above, the conventional method for obtaining directivity based on the principle of wave interference has a number of disadvantages inherent to the basic principle.

The principal object of the invention is to obviate such disadvantages by providing a novel directional wave receiving system having a sharp directivity based on the Doppler effect to substitute for conventional means based on the principle of wave interference.

According to the directional wave receiving system of the invention, a directivity proportional to =cos 9 (wherein n is an integer such "as 0, 1, 2, and 6 is an angle of incidence of the wave to be received) is obtained by frequency modulating the wave to be received in a sinusoidal manner either by vibrating a receiving element to produce a sinusoidal movement thereof or by disposing a plurality ofreceiving elements substantially along a straight line and switching over each receiving element in turn to obtain a frequency modulated sound wave to simulate a condition as if an element is vibrated to make a sinusoidal movement, and then separating suitable components of the thus frequency modulated signal in order to utilize those side band waves which correspond to the desired direc-tivity.

The directional wave receiving system of the invention is characterized to have the following four advantages.

(a) Wide band.'lhe directivity is maintained constant over a wide frequency range, Le. a uniform directivity is obtained for large specific band. (Wide band characteristics.)

(b) Free from side lobes-The system does not produce any side lobes. (No side lobe characteristics.)

(c) Controllable rlireetivity-The sharpness of the main lobe is easily controllable. (Controllable directivity.)

(d) High fidelityr fhe contents of information are not deformed at all in the course of transmitting or receiving, said information through the system of the invention. (Fidelity characteristics.)

The need for the above characteristics may be apparent from the preceding descriptions, however, supplcme .tary explanations will be made hereinafter with regard to the item (d). For instance, in case of sound waves, we may obtain a microphone having a directivity proportional to cos 0, and by using :2 units of such microphones and merely by multiplying the output signal therefrom by each other in succession, a sharp directivity proportional to cos 6 can be obtained. By such a manner, the characteristics (a), (b), and (c) in the above list can be attained by such a combination of conventional microphones, however, the high fidelity according the above item (d) is lost due to the fact that the non-linearity inherent to each multiplier causes distortion of the signal. In most conventional directional systems, a heavy stress has been placed on the item (d) of the above characteristics to make it the primary requirement While making the remaining characteristics (a) to (c) more or less secondary requirements, and hence, there has been hardly a single microphone system provided with such characteristics as (a) to (c) simultaneously with the high fidelity (d). On the other hand, according to the directional wave receiving system of the invention, all of the above four characteristics are achieved at once in a single receiving system.

The inventor has ascertained that if a sound wave is received while moving a microphone, the sound wave thus received is frequency modulated due to the well known Doppler effect and the amplitude of the side band waves in the thus frequency modulated sound wave depends on the direction or the incident angle of the incoming sound wave, and further found that directivity of any desired degree can be attained by discriminating and utilizing that side band of said frequency modulated sound wave which corresponds to the desired degree of directivity, and thus invented the novel directional wave receiving system based on such ascertainments and discovery.

For a better understanding of the invention, reference is taken to the accompanying drawings, in which:

FIG. 1 is a diagrammatical arrangement illustrating the operative principles of the directional wave receiving system of the invention based on the Doppler effect;

FIG. 2 are curves representing values of Bessels function I (x) of the order n, which is related to the intensity of different frequency components produced by the Doppler effect;

FIG. 3 illustrates curves showing the side band wave spectrum of a sound wave, which is received and frequency modulated by taking advantage of the Doppler effect;

FIG. 4 is a block diagram illustrating the circuitry of the directional wave receiving system of the invention in the case of moving a receiving element;

FIG. 5 is a block diagram illustrating the circuitry of the directional Wave receiving system in the case of using a plurality of stationary receiving elements disposed substantially on a straight line and switching over each said receiving element in turn by means of an electrical system;

FIG. 6 shows curves representing spectrums produced 42 by pulse amplitude modulation in the embodiment of FIG. 4;

FIG. 7 is a diagrammatic arrangement of an example of linear disposition of a plurality of receiving elements in the embodiment of FIG. 5;

FIGS. 8a and 8b are diagrams showing the relationship between the position of receiving elements and switch-over signals in an electronic switch-over system; and

FIG. 9 is a similar diagram to FIGS. 8a and 8b, showing the relationship between positions of the wave receiving elements and switch-over signals.

Referring to FIG. 1, the principle of the present invention will be explained. In FIG. 1 it is assumed that a sound collecting point M, which may be a microphone element vibrated in the Y axis at a velocity (V sin at), wherein V is the maximum instantaneous velocity of the vibration, a the angular frequency thereof, and t time. In FIG. 1, S is assumed to be a plane wave having an angular frequency W and an incident angle 0 to the Y axis.

Under the assumption that the signal wave S is frequency modulated in a sinusoidal manner by the vibration of the microphone due to the Doppler effect, then the instantaneous angular frequency W of the output from the microphone is given by =ll l+ g cos 6 sin at) (I) here, C is the velocity of sound, W the angular frequency of the signal wave and A the wave length of the incident signal wave. Accordingly, the output from the microphone is given by e E sin (W t-l-mf sin at) (2) where mf is a modulation index and may be formed by By extending the Forumla 2 in Fouriers series, one obtains 0:1 2 J,,(mf) sin (W zlznafl (1) wherein the Bessels function 1,,(mf) is proportional to (m over a considerably wide range of (inf) as shown in FIG. 2 by substituting (mf) with x for simplicity, and hence one obtains 0 :1? 2 K mf sin (W inafi which is a constant determined by 11.

FIG. 3 illustrates a frequency spectrum of side band waves of the frequency modulated signal thus produced and the signal wave having an angular frequency of W acts as a carrier wave and the mechanical vibration of the microphone produces a wave being frequency modulated by the carrier wave.

It is apparent from the Formula 6 that the FM side waves in said spectrum are produced in groups at an interval of angular frenquency a of the microphone vibration.

In practice, the signal wave S having an angular frequency W corresponds to an amplitude modulated side band lying within a frequency band of 0 to W (W being the maximum frequency of the sound signal), and each frequency element of W appears in each side band of the frequency modulated wave. Since the wave of W =0 is not actually present in acoustic waves, there is no frequency component in the above spectrum at frequencies na (n=1, 2, 3, etc.). This is shown by each of dotted lines at a, 2a, 3a

Therefore, each side band of the frequency modulated wave will correspond to both a side band signal of an amplitude modulated wave and an eliminated carrier wave.

If the angular frequency a is made larger than 2W i.e. a 2W then each group of the side band waves is not overlapped by each other in the frequency spectrum.

As shown in the Formula 6, the amplitude of each frequency component in the side band of the frequency modulated wave is a function of the angle 0 of incidence of the sound signal, the directivity of the microphone is varied depending on the power index n in cos 0. Accordingly, the desired directivity can be obtained by deriving a side band wave which corresponds to the desired power index n through a suitable band-pass filter.

According to the directional wave receiving system of the invention based on the Doppler effect, the directivity,

thereof depends only on cos 0 but not on the frequency, as apparently understood by Formula 6. Thereby, the wide band characteristics, or the item (a) as described in the foregoing with regard to the features of the invention, is achieved to a satisfactory extent. Besides, mathematically speaking, there is no side lobe in cos 0, so that the directivity of the system of the invention is free from side lobes, as described in the item (b) of the feature thereof. However, there are two main lobes at the front side and at the back side due to the symmetry of cos 0, nevertheless there is no side lobe at all. The directivity or the sharpness of the lobe of the system of the invention is also adjustable by controlling the power index n of cos 0 (n being 1, 2, 3, etc.) by such operation as switching a filter circuitry to derive the desired side band of desired power index n, thereby the item (c) of the feature, which is adjustable directivity, is fulfilled. Furthermore, the Doppler effect is caused only between the frequency of the incoming signal and the frequency of vibration of the wave receiving elementary vibration such as a microphone. No Doppler effect is caused between different frequency components of the incoming signal even when said incoming signal consists of a plurality of different frequency components or a series of continuous frequency components, thereby the item ((1) of the feature of the system of the invention, which is high fidelity, is also satisfied.

Thus, a novel directional wave receiving system having a number of outstanding features can be achieved by taking advantage of the Doppler effect. Then, the de sired signals such as sound signals can be reproduced by using a conventional system for demodulating said side band waves having no carrier wave.

Judging from the Formula 6, the amplitude of each FM side band wave is given by Accordingly, in a certain direction for instance 0:0 as the frequency W of a signal is increased, the amplitude of said signal having said frequency is also increased in each group of the side band of order n.

Therefore, it is necessary to compensate the low frequency components of the thus obtained sound signal. Since the sensitivity for the side band of nth order is increased in proportion to nth power of the frequency W the incoming signal Wave, an equalizer attentuating the high frequency band by 6n db/ octave is necessary in order to obtain fiat sensitivity-frequency characteristics over a desired frequency band.

Such an equalizer can be obtained by connecting n units of an RC integrating circuit in series, however, the practicable range of the level for such equalization is limited by the dynamic range of the amplifiers, thereby the frequency band in which the desired flat sensitivityfrequcncy characteristics are achieved with satisfactory S/N ratio is also limited. Such frequency band is liable to become narrower as the order n is increased (see FIG. 2).

Such difi'iculty can be obviated by dividing the desired frequency band into a plurality of subdivisions, and assigning to each of the thus divided subdivisions a suitable value of displacement amplitude d to be described hereinafter. For instance, if a sound signal consisting of frequency components ranging from 0.05 to 5 kc. is di-. vided into two subdivisions, i.e. a first subdivision for 50 to 500 c./s. and a second subdivision for 500 to 5,000 c./s., and furthermore, a microphone assembly having a displacement amplitude of ti; is used for the fist subdivision and another microphone assembly having a dis placement amplitude one tenth of the above d then the modulation index for 5,000 c./s. will become the same with that for 500 c./s., and accordingly, the sensitivity for the two frequencies will become the same. Thereby, the S/N ration for the entire frequency range of 50 to 5,000 c./s. is improved.

If it is desired to achieve satisfactory directivity for an order higher than n=2, a large value of x is used (together with a large d) in order to prevent attenuation of high order signals. Consideration similar to the above should be made with regard to the equalizer of this case too.

In the case of x=2 (which corresponds to comparatively high portions of the linear sections of the Bessels function I (x) of the order n=2 and n=3, as shown in FIG. 2), the value of d can be determined in the following manner. From the Formula 6, one obtains 1; 00a Go here if it is further assumed that C =332 m./sec. W =21rX 10,000 rads/sec., then, d=20.8 rnm.

If the frequency range is divided so as to bring about W =21r 1,0U0 rads/sec., then d=208 mm.

It is also apparent from the Formula -6 that the amplitude of the FM side band wave is reduced as the order n thereof is increased. Hence, if different side bands of different orders n are going to be used for the purpose of adjusting the directivity, it is necessary to compensate their amplitudes to keep them unchanged. Such compensation can be achieved by changing the displacement amplitude d of the microphone.

Changing the displacement amplitude d corresponds to changing the frequency deviation due to frequency modulation, so that the modulation index can be increased by increasing the displacement amplitude d. For instance, in order to achieve a constant amplitude when the order n is changed in the case of W =21r 5,000' rad. see. the displacement amplitude d should be selected according to the following table.

'IL "l 1 2 11 (mm.) u 4 microphone M in a direction of the desired sound source at a frequency of a/Zr. The output of the microphone M is fed to an amplifier i. If the frequency of the incoming signal wave with incident angle a with the direction of vibration is assumed to be W then the output from the microphone will have a spectrum expressed by W ina (/z is positive integer l, 3., 3, etc.). Thereafter, a spectrum component which corresponds to the desired order n, which is adjusted by knob 9, and the value of which expresses a desired damping character in the direction 0, is separated by passing the amplified wave through a bandpass filter 2 which, for instance, consists of contiguous band pass filters made of L.C.R. elements and demodulated at a demodulator 3 by a carrier having an angular frequency of ml, which is fed from the oscillator 6 through a frequency multiplier 7, which is also adjusted by knob 9 so as to make 11 time multiple of the frequency of oscillator 6, and an amplifier 8, thereby the desired signal wave having the same angular frequency with that of the incident signal wave is produced at the output from an audio band low-pass filter 4. Said output from the low-pass filter 4. is provided with a directivity pattern proportional to nth power of a cosine of the angle 6 of incidence formed between the direction of the vibration of the microphone M and the incoming direction of the signal wave under consideration. As is explained with reference to Formula 6, by varying the power index n of cos 0, the character of directivity varies; therefore, the output can be controlled to produce a desired direction. The sharpness of the directivity can be controlled by selecting the power index n by the adjustment of gang controlling knob 9 thus to modify the pass-band of the band-pass filter 2 and the number of multiple of the multiplier 7.

Since it is very ditficult to vibrate a microphone in the field of signal wave in practice, a plurality of elementary microphones are placed in the field in order to achieve the same effect with the mechanical vibration thereof by switching over thus placed stationary microphones in a suitable manner. The method for such switchingover will now be explained in detail with reference to FIGS. and 7.

The operative principles of such operation will be described by taking the case of receiving a signal wave having a spectrum spreading over a frequency range of 0 to W as an example, and the same operative principles are applicable for receiving a different signal wave having a spectrum spreading over a frequency range of V1 (0 W so that the same Wave receiving device may be modified with ease to receive the latter signal wave.

In order to achieve the directivity of order n proportional to cos 0, it is necessary at first to separate those side band waves which are in the frequency band of W +na l V W +na from the rest of the entire frequency spectrum. The sampling frequency v necessary for attaining the required directivity by successively switching over a plurality of receiving elements should satisfy the following formula according to the well known sampling theorem.

The ratio k between the left and right sides of the Formula 7 should be an integer which is large enough not to cause any intereference on the signal elements in the frequency range of -W +na W V +na by side band spectrum carried by higher harmonics of said sampling frequency. The actual value of the ratio k can be selected to suit the need of each designer. FIG. 6, illustrates an example of frequency spectrum of signal elements thus obtained. In FIG. 6, in addition to side band waves due to the Doppler etfect, other side band waves carried by a carrier wave 1711/ (m is an integer). It is necessary to select a sufficiently large 1 for suppressing the interference due to such undesirable side band waves k='(k /2)+1, if k, is even (8) FIG. 7 illustrates an example of disposition of said receiving elements, wherein all elements are aligned on the Y axis, and the ordinates y of rth wave receiving element should satisfy the following relation.

kz /z n+1 if k, is odd I (rl) ,l/ d sin (21 kt Here,

FIG. 8a illustrates an example of the disposition of such receiving elements in the case of k=4 and k =6, and FIG. 8b the same in the case of k=4 and k =7. With such an arrangement, it is made possible to sample at a sampling interval of 1/ 11 the wave receiving system simulating a vibration having a vibrating speed of Va sin at.

In other words, assuming that e e c are receiving elements disposed as shown in FIG. 7 (wherein k; is even), the output signals m m m from each of said receiving elements respectively are led to the pulse amplitude modulator (PAM) 11 of the directional wave receiving system as shown in FIGv 5, as input signals thereto in order to apply pulse amplitude modulation at the sampling frequency 1/ cycle/second determined by the sampling pulse oscillator 16. If the ratio k is even, then said output signals are scanned in the sequence of m m m m m m I71 1 n'l 2 I713, I112, I711, I712, I713, and SO on in a cyclic manner, on the other hand, if the ratio k is odd, then in the sequence of m m m m' l71 1 1711;, In ln 1), 1710;,42), I713, I712, I111, I711, m m and so on in a cyclic manner.

By passing the output from the PAM 11 through a band-pass filter 12, those frequency elements thereof which fall in the frequency range of W +n2z W W -|-na in the frequency spectrum are separated as shown in the Formula 6. The separated elements are apparently that side band which has a directivity of cos 0. Accordingly, by demodulating said separated elements in a demodulator 13 with a carrier of na/211- cycles/ second supplied from the carrier oscillator 17 and passing through the lowpass filter 14, such output signal of the same frequency with that of the incoming signal wave will be achieved at a directivity proportional to cos 0.

The above output signals having a directivity proportional to cos 0 are provided with a pre-emphasis in direct proportion to W as shown in the Formula 6, therefore the equalizer 15 having frequency characteristics in direct proportion to W is inserted in order to achieve flat over all frequency characteristics when the entire system from the wave receiver to the output side of the equalizer is considered. Such equalizer can be made by connecting in series 11 units of high frequency attenuators of 6 db/octave. Alternately, it is possible to insert the equalizer 15 at any desired location between the receiving element e and the lowpass filter 14.

As described in the foregoing, there is a following relation between the angular frequency a of the carrier oscillator 17 and the frequency v of the sampling pulse oscillator 16.

2vrv/a k k being an integer (12) Therefore, it is preferable to provide a synchronism between the two oscillators.

In order to minimize the harmonics interference due to side band waves carried by a carrier having a frequency equal .to multiples of the sampling frequency, it is preferable to carry out the sampling and rectangular wave demodulation in case of pulse amplitude modulation.

Thus, the desired directivity is achieved by disposing a plurality of wave receiving elements, such as miniature microphones, in a row, .and sampling the output in a sinusoidal manner with respect to time and space. It has been also described that the same effects can be achieved by aligning a plurality of wave receiving elements on a certain line and by switching over each of thus aligned receiving elements in turn in a suitable manner. A numerical example has been taken for further detailed description of the invention. I

The disposition of the wave receiving elements for simulating the sinusoidal Doppled effect is not limited to the examples described in the foregoing, as will be shown hereinafter.

The relation between the positions of wave receiving elements and the switching signals in the case of utilizing an electronic switching system will now be described.

The disposition of the wave receiving elements as illusstrated in FIGS. 8:: and 8b are prepared by assuming that the time interval between succeeding switching signals for actuating switch-over operation of the wave receiving elements is uniform, in other words, the repetition frequency of the switching operation is constant, i.e. k=4 and k =6 in the case of FIG. 8a and k=4 and k =7 in the case of FIG. 8b. The dispositions of receiving elements, however, are not limited to the above ones judging from the operative principles thereof.

Since the invention is based on the Doppler effect when a wave receiving element is subjected to a sinusoidal vibration along a certain definite direction, any combination of wave receiving element positions and switching signals may be permissible as long as said combination is equivalent to said sinusoidal vibration of the receiving element.

Referring to FIG. 9, if it is assumed that a vector r has a magnitude of d(lr]=d) and rotates at a constant angular velocity of a, then the isometric projection y of the top end of the vector r on the Y axis is given by which represents a sinusoidal vibration of the wave receiving element. As it will be evident from FIG. 9, in the case of preceding examples, it is assumed that the sampling is made at a uniform time interval and the geometrical distances between wave receiving elements are so selected as to satisfy the relation of the Formula 1.

On the contrary, it is also permissible to divide the geometrical distance y uniformly and to divide the time so as to satisfy the following relation.

y=d sin at 1 y (y) a 1!) A sampling pulse oscillator to generate switching signals at suitable time intervals satisfying the relation of the Formula 1" can be manufactured with case.

In addition, any other combination of the position of wave receiving elements and the switching signal timing will be permissible provided that said combination satisfies the relation of the Formula 1'. The sampling intervals, however, should be so selected as to always fulfill conditions for satisfying the sampling theorem, which is an essential requirement for successfully expanding a continuous wave form into a series of discrete numerals.

As described in detail in the foregoing, the directional wave receiving system of the invention based on the Doppler effect is characterized in that:

(1) Directivity proportional to cos 0 (n is an arbitrary integer) is achieved;

(2) Accordingly, no side lobes are produced;

(3) Directivity pattern thereof is independent of the frequency of the incoming signal wave;

(4) No distortion is produced either by harmonics or .by mutual modulation even when the incoming signal conman, Co a C0 What I claim is:

1. A directional microphone system comprising a plurality of sound wave receiving elements aligned along an axis in spaced relative positions corresponding to ordinates having sinusoidal relation, means to successively switch over said elements in reciprocative manner to sample each output of the element at a uniform interval to cause simulated sinusoidal oscillation along said axis to cause frequency modulation by the Doppler effect of an incoming signal having an incidental angle 6 with the axis, pulse signal oscillator means to generate switching pulses for the successive switching of said elements, signal oscillator means coupled with the switching pulse signal oscillator means to determine the reciprocative frequency, bandpass-filter means for separating an nth side band wave of said frequency modulated signal wave which corresponds to a desired directivity, adjusting means to simultaneously control said band-pass filter means and said signal oscillator means to obtain an nth multiple signal of an output signal of the signal oscillator means to adjust the desired directivity, means for demodulating said separated nth side band wave by means of the nth multiple signal, and lowpass-filter means to remove high frequency components of the switching signal to provide a signal having a directivity proportional to cos 0, wherein n=0, 1, 2,

References Cited UNITED STATES PATENTS 2,305,626 12/1942 Lee 340-100.10 3,055,002 9/1962 Waller 343-1 13.2 3,104,391 9/1963 Hansel 343-1132 2,411,518 11/1946 Busienies 3431l3.2 2,557,979 6/1951 Labin 34318 3,234,554 2/1966 Earp et al 343l06 RODNEY D. BENNETT, Primary Examiner.

C. E. WANDS, Assistant Examiner. 

